ante meridiem

mid-14c., "noon, midday," from Old French meridien "of the noon time, midday; the meridian; a southerner" (12c.), and directly from Latin meridianus "of midday, of noon, southerly, to the south," from meridies "noon, south," from meridie "at noon," altered by dissimilation from pre-Latin *medi die, locative of medius "mid-" (from PIE root *medhyo- "middle") + dies "day" (from PIE root *dyeu- "to shine").

The cartographic sense of "a great circle or half-circle of a sphere passing through the poles" is attested from late 14c., originally astronomical. Figurative uses tend to suggest "point of highest development or fullest power," implying a subsequent decline. As an adjective from late 14c. Related: Meridional. The city in Mississippi, U.S., was settled 1854 (as Sowashee Station) at a railway junction and given its current name in 1860, supposedly by people who thought meridian meant "junction" (they perhaps confused the word with median).

also a.m., 1762 in reference to hours, an abbreviation of Latin ante meridiem "before noon" (q.v.). Synonymous with "morning" by 1776. AM as a type of radio wave broadcast, 1921, abbreviation of amplitude modulation (see amplitude). Affixed to a name, an abbreviation of artium magister "Master of Arts," an abbreviation preferable in a purely Latin idiom; British English preferred M.A. (magister artium, 1730). In some old chronologies, a.m. is anno mundi "year of the world."